This invention relates to digital signal reproducing apparatus and, more particularly, to such apparatus which is readily adaptable for use with a signal playback arrangement that may exhibit different transmission characteristics but, nevertheless, minimizes the error rate of the played back signals notwithstanding such different transmission characteristics.
In the field of video signal recording, a so-called 8-mm recording format has become popular. In 8-mm recording, both video and audio signals are recorded in slant tracks by rotary heads, with one field of video signals recorded in each track. Consistent with a standard which has developed, video and audio signals are frequency multiplexed for recording in a common track.
A typical frequency spectrum used with 8-mm recording is illustrated in FIG. 1. As shown, the frequency multiplexed signals are comprised of the combination of a frequency modulated luminance signal SF, a frequency-converted chrominance signal SC, FM audio signals SM and SS and pilot signals SP. The frequency modulated luminance signal is formed of a carrier which is frequency modulated with the luminance component SY such that the frequency representing the sync tip level is on the order of 5.7 MHz and the frequency which represents the white peak level of the luminance component is on the order of 7.7 MHz.
The frequency-converted chrominance signal is formed by frequency-converting the chrominance subcarrier to a relatively low range below that of the frequency modulated luminance component. The technique of providing a frequency modulated luminance component and a frequency-converted chrominance component has been long known to those of ordinary skill in the art.
The FM audio signal SM is comprised of a carrier that is frequency modulated with the sum (L+R) of the left-channel and right-channel stereophonic audio signals; and the frequency modulated audio signal SS is comprised of another carrier that is frequency modulated with the difference (L-R) between the left-channel and right-channel stereophonic signals. It is appreciated that, on reproduction, good stereophonic quality can be recovered from the FM audio signals SM and SS that have been recorded in a slant track.
The pilot signals SP are used for tracking servo control when the frequency-multiplexed combined signals are reproduced. As is conventional in 8-mm recording, a respective pilot signal whose frequency is one of four different pilot frequencies is recorded in each track such that when a playback head picks up a pilot signal from an adjacent track, the magnitude of that picked up signal is indicative of the tracking error of the head and the frequency of that signal represents the direction in which the head has drifted from its desired position. Thus, tracking control is attained as a function of pilot signal cross-talk components which are picked up when a given track is scanned.
FIG. 2 is a schematic illustration of slant tracks recorded on a magnetic tape 2 in the 8-mm format. Typically, a pair of diametrically opposed rotary heads is used to scan respective tracks across tape 2 which is wrapped about a tape drum with a wrap angle on the order of 221.degree.. If one video field is recorded in one track, the heads are driven at the frame rate to reproduce a video frame at each complete rotation. The combined frequency-multiplexed FM luminance and converted chrominance components as well as the FM audio signals are recorded along an angular extent of 180.degree. in that portion of each track identified as the video portion 2V. To improve the audio quality of 8-mm recording, it is optional also to record audio signals in a separate portion of each slant track shown in FIG. 2 by the cross-hatched sections identified as the audio portion 2A. These audio signals are recorded as pulse code modulated (PCM) audio signals in an angular extent of about 41.degree. and may represent the same audio information as the FM audio signals recorded in video portion 2V (but with better quality). Thus, to record the slant tracks illustrated in FIG. 2, a PCM audio signal is recorded for about 41.degree. and then the frequency-multiplexed combined video and audio signals are recorded in the remainder of the track (e.g. for about 180.degree.). Even if the very same audio signals are recorded as both FM audio signals SM and SS and as PCM audio signals, the quality of the PCM audio signals is far superior. Nevertheless, the FM audio signals are recorded in portion 2V as part of the frequency-multiplexed combined signals in order to be compatible with typical 8-mm standards.
Typically, the PCM audio signals are produced by quantizing left and right channel audio channels into digital signals DD having 16 bits per sample at a sampling frequency of 48 kHz or 44.1 kHz or 32 kHz (each of which sampling frequencies has been adopted as a modification of the 8-mm recording standard). The digitized audio signals may be encoded in an error correcting code, such as a BCH code, or may have an error correcting code added thereto, such as a parity character, ECC code or the like. Then, the digitized audio signals are timebase compressed and converted in accordance with conventional recording code conversion techniques, such as 8-to-10 conversion. The resultant digitized, error-corrected, compressed and converted audio signals are recorded in audio portion 2A of each slant track.
The PCM audio signal recorded as aforementioned exhibits high quality and may be reproduced with characteristics equal to or better than the characteristics of audio signals reproduced by typical CD or R-DAT formats.
When reproducing digital signals, such as the PCM audio signals reproduced from the magnetic tape shown in FIG. 2, the use of equalizing circuitry is important to the elimination of intercode interference and to permit the demodulation of the left-channel and right-channel audio signals with minimal error rate. However, proper equalization generally is dependent upon an accurate prediction of the transmission characteristics exhibited by the signal transmission system formed of the record medium, the playback head, the conducting leads and the signal processing circuitry. Usually, these transmission characteristics vary from one playback device (or 8-mm recorder) to another. Since the transmission characteristics depend to a significant degree upon the particular magnetic tape that is used, the availability of a wide variety of tape practically assures different transmission characteristics each time a different tape is used. Thus, and with reference to FIG. 3, even if an equalizing circuit having an optimum equalization characteristic is selected, such as the characteristic indicated at point 2 for a particular tape, when other tapes are used the very same equalizing circuit may exhibit the relative equalization characteristic indicated at points 1, 3 or 4. Hence, although an optimum error rate may be obtained for this equalizing circuit when magnetic tape corresponding to point 2 is used, poorer (i.e. higher) error rates may result when magnetic tapes corresponding to points 1, 3 or 4 are used.
The problem of matching equalization characteristics to transmission characteristics, and particularly to the type of tape and head which are used to reproduce digital signals may be overcome by using automatic equalizing circuits. However, such automatic equalizing circuits are complex and expensive and may contribute to a significantly higher overall cost of the 8-mm recorder.
Furthermore, the most favorable equalizing characteristics generally are obtained from an automatic equalizer circuit by executing a sequential equalizing process to arrive at the most favorable error rate. This sequential process is graphically depicted in FIG. 4 wherein different equalizing characteristics are selected in sequence, while sensing the error rate resulting from each selected characteristic. Once this sequence has been performed, the equalizing characteristic which resulted in the most favorable error rate is selected. However, this sequential process is time-consuming, and it may take several seconds until the most favorable equalizing characteristic is selected. Thus, there is a substantial and undesired time delay from the time a user first operates an 8-mm recorder to carry out a playback operation until satisfactory audio quality is obtained.